The present invention pertains to color balancing of photographic printing paper.
Although the present invention has application in various photographic processes, it is particularly useful with processes wherein a positive print is produced from a color negative using materials known as integral tripacks which have three superimposed light-sensitive emulsion layers. Those skilled in the art will appreciate that it is rarely if ever possible to produce a print with accurate color rendition without adjusting for color-balance variations, which are caused by color-mix changes in lighting conditions and changes in the characteristics of films and printing papers. Negative-positive processes provide substantial latitude in making color correction in printing the negative by selective variation of red, green, and blue light exposures. There are two principal techniques for making such exposure variations in present use. In tricolor additive printing, three separate exposures of the negative to the printing paper are made respectively through red, green, and blue separation filters, wherein the three exposure times are varied to control color correction. Exposure of the printing paper to varying amounts of red, green, and blue light, hereinafter referred to as the primary colors of light, is thereby achieved. In white-light subtractive printing, a single exposure of the negative to the printing paper is made through a combination of colored filters, wherein filter densities and colors are varied to control color correction, thereby selectively filtering out varying amounts of the primary colors of light. In either case, color control is achieved by selectively varying the exposures of the primary colors of light to the printing paper, exposure being basically a product of light intensity and duration. Such colored light variations produce corresponding variations in the production of complementary colored dyes by the three emulsion layers, red light producing cyan dye, green light producing magenta dye, and blue light producing yellow dye.
Most prior art color balancing techniques employ visual color judgments using test prints. Of particular application to the white-light subtractive printing technique is the use of color filters for viewing a test print to make a judgment as to filtering changes for making subsequent prints. Less subjective techniques involve the use of electronic color analyzers to detect changes in color balance from negative to negative. Proper use of such color analyzers requires information as to the color balance of the printing paper being used. Due to changes in storage conditions and age, no two boxes of color printing paper have precisely the same color balance. Therefore, upon starting a new box of printing paper, a test print is made to obtain the necessary color balance information, which can then be used to reprogram the color analyzer. Here again even the most sophisticated prior art techniques resort to subjective visual judgment in obtaining such color balance information about the new printing paper to be used. In one popular technique, a picture is first taken of a gray card, whereupon the gray card negative is printed through a matrix of colored filters onto a sample sheet of the printing paper under test. By visual comparison, a gray patch is then found on the print, enabling the selection of an appropriate filter combination for color balancing the system. The color analyzer can then be programmed to such a filter combination to compensate for the characteristics of the printing paper. Different color negatives can then be tested for color balance by the color analyzer, enabling exposure corrections to be made regardless of whether tricolor additive or white-light subtractive printing is then used.
Subjective visual judgment is eliminated in one prior art technique of ascertaining the color balance of photographic printing paper in accordance with U.S. Pat. No. 3,392,626. While the procedure is effective, it is also laborious and time consuming. It requires measuring reflection densities corresponding to discrete values of step wedge density for each of three color exposures, then plotting graphs from the density measurements, and finally making numerical calculations from the graphs to provide color balance data.
The present invention provides a greatly simplified objective instrumental technique for obtaining the color balance of photographic printing paper without reliance on subjective visual judgment and without the laborious graphing techniques described in the cited patent.